/**
*      author: Y_Yao
*      created: 2025-04-09 21:27:24
**/

#include<bits/stdc++.h>

#define int LL
using namespace std;
using LL = long long;
const LL LLINF = 9e18;
const int INF = 0x3f3f3f3f;
int n,m;

void solve()
{
    cin >> n >> m;
    if(n == 0 || n == 1)
    {
        cout << 0 << endl;
        return;
    }
    if(m == 1 || m == 0)
    {
        cout << -1 << endl;
        return;
    }
    if(m == 2)
    {
        cout << n - 1 << endl; 
        return;
    }
    int tm = m;
    int ans = 1;
    while(tm < n)
    {
        tm += m - 1;
        ans++;
    }
    cout << ans << endl;
    // cout << (n - 1 + m - 2) / (m - 1) << endl;

}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);

    int t = 1;
    // cin >> t;
    while(t--)
    {
        solve();
    }
    return 0;
}